Glass fibers are now produced for use in a broad variety of technologies. Their production involves a procedure wherein molten glass is passed through a multiple orificed bushing at a relatively high rate. As the very small nascent glass streams extend from the bushing, they encounter a "prepad" or cooling stage usually provided as a water spray. The cooled and now rigidified "filaments" then encounter a "pad" or size applicator at which point they are coated with a protective film, following which the coated filaments are brought together at a "gathering shoe" to form a "strand". This strand is coiled upon a spool to form a "package". During formation of the package, the strand is traversed back and forth across the spool by a spooling cam driven device or the like, often referred to as a traverse. The spool is rotated at a collet or winder and the tension exerted upon the strand by the winding activity serves to attenuate the molten stream of glass passing immediately from the bushing.
The above-described sizing procedure is highly important to the successful production of strand as well as to the formation of the great variety of products utilizing glass fibers. In this respect, when considered in isolation, glass filaments have a relatively high theoretical strength. However, in the course of their formation to strand and subsequent glass fiber products, such strength is not realized, due at least in part to minute defects developed upon filament surfaces occasioned by glass on glass abrasion during production. A highly important function of the sizing procedure therefore, is to impart a desired degree of lubricity to the filaments, thus lessening this abrading phenomena. Filament degradation further may be influenced by the water content of sizes with which they are coated, the most conventional of sizes being provided as aqueous solutions or emulsions. See in this regard, U.S. Pat. No. 3,473,950.
Another important function of the size coating stems from the nature of the glass filament surfaces. These surfaces are very smooth and are highly hydrophilic in nature. In consequence, a thin film of water tends to form upon the filament surfaces functioning to disrupt any bond, chemical or physical, which would otherwise be formed between the glass and the material within which it may be embedded. Accordingly, in addition to the above described lubricant, the size generally will incorporate a coupling agent serving to react both with the silica in the glass as well as with organic matrix material within which the filaments and strands usually are embedded. As is apparent, the selection of coupling agent looks to the ultimately intended use of the fiber, a compatible coupling agent of one variety being utilized with elastomers and the like, while that of another variety being utilized with glass fiber reinforced resinous articles and the like.
Film formers provide the final principal active ingredient of the sizes and are selected as resins of relatively higher molecular weight, for example epoxy resins and the like. The film formers serve in effect to provide a tougher film coating which, for many applications of the glass fibers, imparts a necessary integrity or character to strand. For example, where the strand is utilized as roving for weaving glass fiber reinforcing sheets and the like, a certain stiffness is required of the product to permit its manipulation by the ultimate user in connection with placement within molds and the like. The strand stiffness translates into the woven integrity of the cloth to permit its facile manipulation. Of course, the selection of the stiffness quality must be such as to derive adequate cloth integrity while still permitting manipulation of the cloth about the corners of a mold or the like within or upon which it is utilized.
In general practice, the coupling agent, lubricant and film former are applied to the glass filaments utilizing the media of a water carrier. In this regard, the combined "solid" ingredients represent about 5% by weight of the liquid size which is continuously recirculated at the applicator stage from a supply reservoir. Following application of the size, the formation of strands and development of a package as above described, the package is then moved to a drying oven at which point the aqueous or volatile carrier phase of the size is driven off.
To avoid the possibility of attack upon the glass surfaces by the aqueous phase of the sizes, investigators have developed organic carriers or non-aqueous polar solvents as carriers, for example as described in U.S. Pat. No. 3,473,950. Generally, the steps in producing filaments using such sizes follow that described, including the step of heating the completed packages to drive off the volatile organic carrier. In all systems, the carrier represents the major component of the size solution, the solids or coupling agent, lubricant and film formers representing, as noted, about 5% by weight of the material.
A principal difficulty encountered by industry in the utilization of sizes resides in a phenomenon termed "migration". As noted above, the liquid carrier phase of the size is driven off by depositing the strand package in an oven. Inasmuch as these packages typically are cylindrical in form, being wound upon a cardboard collar, the vaporization of the liquid phase takes place somewhat differentially or progressively from the outside and inside faced surfaces as well as the edge surfaces toward the central portion of the package. As the carrier, i.e. volatile constituent, is removed to a point wherein a total 10 to 12 percent moisture content for the package is present, it is opined that migration of the internally disposed carrier toward the outward surfaces commences by capillary action. This causes a movement of the solids component of the size from the position of its initial deposition toward the surface of the package. Such movement of important constituents may result in the development of randomly located regions of the filaments exhibiting an inadequate size coating. Further, there develops an excess accumulation of these important constituents at the externally exposed surfaces of the package.
As indicated earlier, as strand is wound to form a package, it is manipulated in a back and forth fashion through the use of a traverse. Such spool winding teachniques necessarily provide for spaced "turn around" points along the length of each strand. These points represent the strand position at the opposed edges of the cylindrical package wherein the traverse reverses direction of strand. In the course of the above-described migration of liquid carrier and concurrent movement of the important size constituents toward the outer edge of the package, a build-up of the latter constitutents is evidenced at the outer edge surfaces and this build-up then is witnessed as a series of spaced material build-ups in the strand as it is unwound from the package. When the strand is subsequently drawn from the package and woven to form cloth products and the like, the dispersed turn around points with excess size material become visible and represent an undesirable aspect of the resultant woven product. This particularly is true where drying is carried out in conventional fashion within direct gas fired ovens. Where such drying is utilized, the materials of combustion tend to alter the color of the cationic materials usually incorporated within the size. This spotted discoloration tends to derogate from the quality of the resultant product.
As is apparent, the removal of solid size materials due to migration results in dispersed sites of filament surface area having less size. The thus exposed surface areas may exhibit lowered strength characteristics resulting in dispersed filament breakage which may result in undesired "fuzz" formation. Where the sized strand is intended for application as reinforcement within elastomeric product such as pneumatic tires, driving belts, timing belts and the like, it initially is coated with an impregnant. This impregnant serves the purpose of locking the strands within the elastomer and may have a variety of formulations as described, for example, in U.S. Pat. No. 3,850,872. Inasmuch as the impregnant fails to adhere to regions of the fibers which exhibit an excess coating of size, the quality of anchoring of the strands within the elastomer is adversely affected where materials are utilized evidencing the migration effects. Such condition of the impregnant coated strands is generally referred to in the industry as "streaking".
In addition to the pronounced effects of migration in the course of oven drying of the strand packages, the phenomenon also has been observed to commence early in the process of strand formation. For example, migration due to evaporation has been found to occur as strand packages are maintained in holding or surge areas prior to their introduction to the heating step. This migration is caused by the evaporation of the outer surfaces of the package during that holding interval.
The term "glass fibers", as used herein, shall refer to (1) continuous fibers formed by the rapid attenuation of hundreds of streams of molten glass and to strands formed when such continuous glass fiber filaments are gathered together in forming; and to yarns and cords formed by plying and/or twisting a number of strands together, and to woven and non-woven fabrics which are formed of such glass fiber strands, yarns or cords, and (2) discontinuous fibers formed by high pressure steam or air directed angularly downwardly onto multiple streams of molten glass issuing from the bottom side of a glass molting bushing and to yarns that are formed when such discontinuous fibers are allowed to rain down gravitationally onto a foraminous surface wherein the fibers are gathered together to form a sliver which is drafted into a yarn; and to woven and non-woven fabrics formed of such yarns of discontinuous fibers, and (3) combinations of such continuous and discontinuous fibers in strand, yarn, cord and fabrics formed thereof.